Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Detail of regulatory liabilities
Regulatory liabilities, current	$ 135,554	$ 145,766
Regulatory liabilities, non-current	1,009,418	994,152
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	289,485	277,554
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	32,473	39,746
Regulatory liabilities, non-current	244,724	240,367
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	34,100	34,100
Regulatory liabilities, non-current	171,029	179,521
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,774	3,604
Regulatory liabilities, non-current	96,940	97,175
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,771	1,113
Regulatory liabilities, non-current	71,756	72,454
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	31	3,051
Regulatory liabilities, non-current	71,235	67,437
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	41,518	43,773
Regulatory liabilities, non-current	3,274	4,365
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	6,628	6,079
Regulatory liabilities, non-current	19,115	19,115
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	14,080	13,678
Deferred fuel and purchased power (b) (c)
Detail of regulatory liabilities
Regulatory liabilities, current	13,083	9,688
Regulatory liabilities, non-current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,062	2,062
Regulatory liabilities, non-current	5,501	6,001
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	14,725	8,920
Other
Detail of regulatory liabilities
Regulatory liabilities, current	114	2,550
Regulatory liabilities, non-current	$ 7,554	$ 7,565